Hydraulic oil composition

ABSTRACT

The present invention provides a hydraulic oil composition comprising a lubricating base oil having a kinematic viscosity at 40° C. of 15 to 50 mm 2 /s, a viscosity index of 125 or lower, and an aromatic content of 1% by mass or higher; and 1 to 40% by mass based on a total amount of the hydraulic oil composition of a polymethacrylate having a number-average molecular weight of 28000 or lower.

TECHNICAL FIELD

The present invention relates to a hydraulic oil composition.

BACKGROUND ART

In recent years, energy-saving hydraulic oils have been developed as one of responses to global warming. There are some conventional energy-saving hydraulic oils allowing achieving the reduction of energy consumption of apparatuses at starting, for example, by decreasing their low-temperature viscosity.

There are also developed energy-saving hydraulic oils whose viscosity change is made small by blending a viscosity index improver to thereby reduce energy consumption in the steady-state operation after the oil temperature is raised. In the energy-saving hydraulic oils, the oil leakage (internal leakage) from construction machines' characteristic various hydraulic apparatus interiors is prevented by making small the viscosity change (making the viscosity index high) of the hydraulic oils, and the reduction of the energy consumption is achieved (for example, see Patent Literatures 1 to 3).

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open No.     2005-307197 -   Patent Literature 2: Japanese Patent Application Laid-Open No.     2011-046900 -   Patent Literature 3: Japanese Patent Application Laid-Open No.     2012480535

SUMMARY OF INVENTION Technical Problem

In the case of the energy-saving hydraulic oils as described in the above Patent Literatures 1 to 3, however, the high viscosity index of the hydraulic oils causes an increase in the loss due to the plumbing resistance. Hence, even if the energy consumption can be reduced by the internal leakage prevention, there is still room for improvement in the point of improving the energy efficiency of the hydraulic system as a whole.

The present invention has been achieved in consideration of such a real situation, and an object thereof is to provide a hydraulic oil composition enabling both the internal leakage prevention and the plumbing resistance reduction to be compatibly achieved, and enabling the energy efficiency of a hydraulic system as a whole to be improved.

Solution to Problem

As a result of earnest studies, the present inventors have found a composition exhibiting excellent viscosity characteristics compatibly achieving both the internal leakage prevention and the plumbing resistance reduction of a hydraulic system, and this finding has led to the completion of the present invention.

That is, the present invention provides a hydraulic oil composition comprising a lubricating base oil having a kinematic viscosity at 40° C. of 15 to 50 mm²/s, a viscosity index of 125 or lower, and an aromatic content of 1% by mass or higher, and 1 to 40% by mass based on the total amount of the hydraulic oil composition of a polymethacrylate having a number-average molecular weight of 28000 or lower.

The hydraulic oil composition preferably has the ratio (A/B) of (A) a kinematic viscosity (unit: mm²/s) at 80° C. to (B) a high shear viscosity (unit: mPa·s, shear condition: 10⁶/s) of 1.15 or lower.

Advantageous Effects of Invention

The present invention can provide a hydraulic oil composition which enables both the internal leakage prevention and the plumbing resistance reduction to be compatibly achieved, and enables the energy efficiency of a hydraulic system as a whole to be improved.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferred embodiment according to the present invention will be described.

A hydraulic oil composition according to the present embodiment comprises a lubricating base oil having a kinematic viscosity at 40° C. of 15 to 50 mm²/s, a viscosity index of 125 or lower, and an aromatic content of 1% by mass or higher, and 1 to 40% by mass based on the total amount of the hydraulic oil composition of a polymethacrylate having a number-average molecular weight of 28000 or lower.

The lubricating base oil to be used in the present embodiment includes mineral oils, and synthetic hydrocarbon oils. These lubricating base oils can be used singly or in combinations of two or more.

The mineral oil is not especially limited, but examples thereof include paraffinic mineral oils or naphthenic mineral oils refined by subjecting lubricating oil fractions obtained by atmospheric pressure distillation and reduced pressure distillation of crude oils to suitably combined refining treatments including solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid cleaning and clay treatment.

Examples of the synthetic hydrocarbon oil include poly-α-olefins (polybutene, 1-octene oligomers, 1-decene oligomers and the like), alkylbenzenes and alkylnaphthalenes.

The kinematic viscosity at 40° C. of the lubricating base oil is 15 to 50 mm²/s, preferably 20 to 45 mm²/s, more preferably 25 to 40 mm²/s, and still more preferably 25 to 35 mm²/s. When the kinematic viscosity at 40° C. is 15 mm²/s or higher, the case is preferable in the points of being capable of preventing the lowering of the flash point, and of the evaporation. Further when the kinematic viscosity at 40° C. is 50 mm²/s or lower, the plumbing resistance can be reduced.

The viscosity index of the lubricating base oil is 125 or lower, and preferably 120 or lower. Further the viscosity index of the lubricating base oil is preferably 90 or higher, more preferably 100 or higher, and still more preferably 105 or higher. When the viscosity index is 90 or higher, since when the kinematic viscosity at high temperatures is secured, the kinematic viscosity at low temperatures is suppressed in becoming high, the lowering of the efficiency of a hydraulic system can be suppressed.

Here, the “kinematic viscosity” and the “viscosity index” in the present invention mean values measured according to JIS K 2283.

The aromatic content of the lubricating base oil is 1% by mass or higher, preferably 1.5% by mass or higher, and more preferably 2% by mass or higher. When the aromatic content is 1% by mass or higher, the solubility and the viscosity-increasing effect of the polymethacrylate are likely to be improved. The upper limit of the aromatic content of the lubricating base oil is not especially limited, but the aromatic content of the lubricating base oil is, for example, 35% by mass or lower.

The sulfur content of the lubricating base oil is not especially limited, but is preferably 5000 ppm by mass or lower, more preferably 3000 ppm by mass or lower, still more preferably 1000 ppm by mass or lower, and most preferably 300 ppm by mass. When the sulfur content is 5000 ppm by mass or lower, the case is preferable in the points of the oxidation stability and the corrosion resistance.

The content of the lubricating base oil is preferably 40% by mass or higher, more preferably 50% by mass or higher, and still more preferably 70% by mass or higher based on the total amount of the hydraulic oil composition. Further the content of the lubricating base oil is preferably 99% by mass or lower, more preferably 98% by mass or lower, and still more preferably 95% by mass or lower based on the total amount of the hydraulic oil composition. When the content of the lubricating base oil is 40% by mass or higher, the excellent advantages of the hydraulic oil are easily fully exhibited.

The polymethacrylate which the hydraulic oil composition according to the present embodiment contains is a polymethacrylate having a number-average molecular weight of 28000 or lower. Suitable examples of such a polymethacrylate include a non-dispersant polymethacrylate having a structural unit represented by the following formula (1) and a dispersant polymethacrylate having a structural unit represented by the following formula (2).

[In the formula (1), a is an integer of 1 or more, and is an integer such that the number-average molecular weight of the polymethacrylate is 28000 or lower; and R¹ represents an alkyl group having 1 to 22 carbon atoms.]

[In the formula (2), each of b and c is an integer of 1 or more, and is integer such that the number-average molecular weight of the polymethacrylate is 28000 or lower; R² represents an alkyl group having 1 to 22 carbon atoms; R³ represents hydrogen or a methyl group; and X represents a polar group.]

The number-average molecular weight of the polymethacrylate is 28000 or lower, preferably 25000 or lower, more preferably 23000 or lower, and still more preferably 20000 or lower. Further the number-average molecular weight of the polymethacrylate is preferably 2000 or higher, more preferably 5000 or higher, and still more preferably 10000 or higher. When the number-average molecular weight of the polymethacrylate is 28000 or lower, the case is preferable in the point of an effect of improving the high shear viscosity; and when that is 2000 or higher, the case is preferable in the point of an effect of improving the viscosity index.

The content of the polymethacrylate is 1 to 40% by mass based on the total amount of the hydraulic oil composition. The content of the polymethacrylate is preferably 3% by mass or higher, more preferably 5% by mass or higher, and still more preferably 10% by mass or higher. Further the content of the polymethacrylate is preferably 30% by mass or lower, more preferably 25% by mass or lower, and still more preferably 20% by mass or lower. When the content of the polymethacrylate is 1% by mass or higher, the case is preferable in the point an effect of improving the high shear viscosity; and when that is 35% by mass or lower, the case is preferable in the point that the effect corresponding to the cost can be anticipated.

The kinematic viscosity at 40° C. of the hydraulic oil composition is preferably 20 mm²/s or higher, more preferably 30 mm²/s or higher, still more preferably 40 mm²/s or higher, and most preferably 41.4 mm²/s or higher. Further the kinematic viscosity at 40° C. is preferably 80 mm²/s or lower, more preferably 70 mm²/s or lower, still more preferably 60 mm²/s or lower, and most preferably 50.6 mm²/s or lower. When the kinematic viscosity at 40° C. is 20 mm²/s or higher, the case is preferable in the point of the durability of a hydraulic system; and when that is 80 mm²/s or lower, the case is preferable in the point of the friction reduction.

The ratio (A/B) of (A) a kinematic viscosity (unit: mm²/s) at 80° C. to (B) a high shear viscosity (unit: mPa·s, shear condition: 10⁶/s) at 80° C., with respect to the hydraulic oil composition, is preferably 1.15 or lower, more preferably 1.14 or lower, still more preferably 1.13 or lower, and most preferably 1.12 or lower. When the above ratio (A/B) is 1.15 or lower, the case is preferable in the points of the pump efficiency and the plumbing resistance. The lower limit of the above ratio (A/B) is not especially limited, but is, for example, 1.0 or higher.

Here, the “high shear viscosity” in the present invention means a value measured according to ASTM (D4741, D4683, D6616), CEC (L-36A-90).

The hydraulic oil composition according to the present embodiment, in order to more improve its excellent advantages, can further comprise, as required, an extreme pressure agent, an antioxidant, a pour point depressant, a rust-preventive agent, a metal deactivator, a viscosity index improver, an antifoaming agent, a demulsifier, an oiliness agent and the like. These additives may be used singly or in combinations of two or more.

Examples of the extreme pressure agent includes sulfur compounds such as ester sulfides, sulfurized fats and oils and polysulfides, zinc dithiophosphate, and phosphorus compounds, and phosphorus compounds are preferable. The phosphorus compounds specifically include phosphate esters, acidic phosphate esters, amine salts of acidic phosphate esters, chlorinated phosphate esters, phosphite esters and phosphorothionate. The phosphorus compounds are esters of phosphoric acid, phosphorous acid or thiophosphoric acid with an alkanol or a polyetheric alcohol, and their derivatives.

Among the above phosphorus compounds, since higher antiwear property can be provided, phosphate esters, acidic phosphate esters, amine salts of acidic phosphate esters are preferable, and among these, phosphate esters are more preferable. It is preferable that the content of the extreme pressure agent is 0.05 to 5% by mass based on the total amount of the hydraulic oil composition.

Examples of the antioxidant include phenolic compounds such as 2,6-ditertiary-butyl-p-cresol (DBPC), aromatic amines such as phenyl-α-naphthylamine and organometal compounds. It is preferable that the content of the phenolic antioxidant is 0.01 to 2% by mass based on the total amount of the hydraulic oil composition. Further it is preferable that the content of the amine-based antioxidant is 0.001 to 2% by mass based on the total amount of the hydraulic oil composition.

Examples of the pour point depressant are copolymers of one or two or more monomers selected from acrylate esters and methacrylate esters, and hydrogenated substances thereof. It is preferable that the content of the pour point depressant is 0.01 to 5% by mass based on the total amount of the hydraulic oil composition.

Examples of the rust-preventive agent are amino acid derivatives, partial esters of polyhydric alcohols; esters such as lanolin fatty acid esters, alkyl succinate esters and alkenyl succinate esters; sarcosine; polyhydric alcohol partial esters such as sorbitan fatty acid esters; metal soaps such as fatty acid metal salts, lanolin fatty acid metal salts and oxidized wax metal salts; sulfonates such as calcium sulfonate and barium sulfonate; oxidized waxes; amines; phosphoric acid; and phosphate salts. It is preferable that the content of the rust-preventive agent is 0.01 to 5% by mass based on the total amount of the hydraulic oil composition.

Examples of the metal deactivator are benzotriazole compounds, thiadiazole compounds and imidazole compounds. It is preferable that the content of the metal deactivator is 0.001 to 1% by mass based on the total amount of the hydraulic oil composition.

The hydraulic oil composition can further comprise a viscosity index improver other than the above polymethacrylate. Examples of the viscosity index improver include non-dispersant viscosity index improvers such as copolymers of one or two or more monomers selected from various types of methacrylate esters, or hydrogenated substances thereof, polyisobutylenes or hydrogenated substances thereof, hydrogenated styrene-diene copolymers, and polyalkylstyrenes. It is preferable that the content of the viscosity index improver is 0.01 to 15% by mass based on the total amount of the hydraulic oil composition.

Examples of the antifoaming agent are silicones such as dimethylsilicones and fluorosilicones. It is preferable that the content of the antifoaming agent is 0.001 to 0.05% by mass based on the total amount of the hydraulic oil composition.

Examples of the demulsifier include polyoxyalkylene glycols, polyoxyalkylene alkyl ethers, polyoxyalkylene alkylamides and polyoxyalkylene fatty acid esters.

Examples of the oiliness agent include fatty acids, esters and alcohols. It is preferable that the content of the oiliness agent is 0.01 to 0.5% by mass based on the total amount of the hydraulic oil composition.

Examples

Hereinafter, the present invention will be described more specifically by way of Examples and Comparative Examples, but the present invention is not any more limited to these contents.

In Examples 1 to 3 and Comparative Examples 1 to 3, hydraulic oil compositions were each prepared by blending a lubricating base oil and additives in a composition shown in Table 1 and Table 2. In the preparation of the hydraulic oil composition, by regulating the amount of a viscosity index improver to be blended according to its molecular weight, the kinematic viscosity at 40° C. of the hydraulic oil composition was regulated so that the ISO viscosity grade became VG46. The lubricating base oils and the additives used in the Examples and the Comparative Examples are as follows.

<Lubricating Base Oils>

Base oil 1: a solvent refined mineral oil (aromatic content: 30.0% by mass, sulfur content: 2300 ppm by mass, kinematic viscosity at 40° C.: 28.6 mm²/s, viscosity index: 101) Base oil 2: a solvent refined mineral oil+a hydrorefined mineral oil (aromatic content: 13.0% by mass, sulfur content: 600 ppm by mass, kinematic viscosity at 40° C.: 26.6 mm²/s, viscosity index: 103) Base oil 3: a hydrorefined mineral oil (aromatic content: 2.0% by mass, sulfur content: 10 ppm by mass or less, kinematic viscosity at 40° C.: 29.5 mm²/s, viscosity index: 117) Base oil 4: hydrorefined mineral oil (aromatic content: 0.5% by mass, sulfur content: 10 ppm by mass or lower, kinematic viscosity at 40° C.: 36.4 mm²/s, viscosity index: 131) Base oil 5: hydrorefined mineral oil (aromatic content: 0.5% by mass, sulfur content: 10 ppm by mass or lower, kinematic viscosity at 40° C.: 39.6 mm²/s, viscosity index: 130)

Here, the aromatic content was measured according to silica-alumina gel chromatography described in “Separation of High-Boiling Petroleum Distillates Using Gradient Elution Through Dual-Packed (Silica Gel-Alumina Gel) Adsorption Columns,” Analytical Chemistry, Vol. 44, No. 6, (1972), pp. 915-919.

Further the sulfur content was measured according to ASTM D4951, “Standard Test Method for Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectrometry.”

Further the kinematic viscosity and the viscosity index were measured according to HS K2283.

<Viscosity Index Improvers>

A: a polymethacrylate (Evonik Degussa GmbH, JMB3587, number-average molecular weight having a structural unit represented by the formula (1): 20000) B: a polymethacrylate (Henkel Japan Ltd., Kanelube 2091, number-average molecular weight having a structural unit represented by the formula (2): 40000)

<Other Additives>

As other additives, tricresyl phosphate, 2,6-ditertiary-butyl-p-cresol (DBPC) and a pour point depressant were used.

For each hydraulic oil composition obtained in Examples 1 to 3 and Comparative Examples 1 to 3, the kinematic viscosities at 40° C., 80° C. and 100° C. and the viscosity indices were measured according to MS K 2283. Further for each hydraulic oil composition, the high shear viscosity at 80° C. at a shear condition of 10⁶/s was measured according to ASTM (D4741, D4683, D6616), CEC (L-36A-90). A measuring instrument used was a USV (Ultra Shear Viscometer) viscometer, manufactured by PCS Instruments. The results are shown in Table 1 and Table 2.

An HPV35+35 pump test was carried out on each hydraulic oil composition obtained in Examples 1 to 3 and Comparative Examples 1 to 3. Specifically, the rotational torque of the pump was measured under the following test condition, and the total efficiency was calculated. The results are shown in Table 1 and Table 2.

The pump name: Komatsu HPV35+35

The discharge volume+the drain volume: 40 L/min

The pump type: a swash plate type

The oil temperature: 80° C.

The pressure: no load, 35 MPa

The rotation of the pump: 2100 rpm

TABLE 1 Example 1 Example 2 Example 3 Composition base oil 1 balance — — (% by mass) base oil 2 — balance — base oil 3 — — balance base oil 4 — — — base oil 5 — — — viscosity index 13 14.5 13 improver A viscosity index — — — improver B tricresyl phosphate 0.5 0.5 0.5 DBPC 0.5 0.5 0.5 pour point depressant 0.3 0.3 0.3 Properties kinematic viscosity at 46.29 46.58 46.47 40° C. (mm²/s) kinematic viscosity at 12.97 13.19 13.19 80° C. (mm²/s) kinematic viscosity at 8.24 8.40 8.41 100° C. (mm²/s) viscosity index 153 158 159 high shear viscosity 11.63 11.84 11.64 at 80° C. (mPa · s) kinematic viscosity at 1.12 1.11 1.13 80° C./high shear viscosity at 80° C. Total Efficiency (%) of 66.6 66.8 66.6 HPV35 + 35 Pump Test

TABLE 2 Comparative Comparative Comparative Example 1 Example 2 Example 3 Composition base oil 1 — — — (% by mass) base oil 2 — — — base oil 3 — — balance base oil 4 balance — — base oil 5 — balance — viscosity index 9 — — improver A viscosity index — 6 11.5 improver B tricresyl 0.5 0.5 0.5 phosphate DBPC 0.5 0.5 0.5 pour point 0.3 0.3 0.3 depressant Properties kinematic 46.48 45.73 45.84 viscosity at 40° C. (mm²/s) kinematic 13.18 13.10 13.68 viscosity at 80° C. (mm²/s) kinematic 8.39 8.37 8.84 viscosity at 100° C. (mm²/s) viscosity index 158 161 176 high shear 11.43 11.20 11.45 viscosity at 80° C. (mPa · s) kinematic 1.15 1.17 1.20 viscosity at 80° C./high shear viscosity at 80° C. Total Efficiency (%) of 66.4 66.1 66.4 HPV35 + 35 Pump Test 

1. A hydraulic oil composition comprising: a lubricating base oil having a kinematic viscosity at 40° C. of 15 to 50 mm²/s, a viscosity index of 125 or lower, and an aromatic content of 1% by mass or higher; and 1 to 40% by mass based on a total amount of the hydraulic oil composition of a polymethacrylate having a number-average molecular weight of 28000 or lower.
 2. The hydraulic oil composition according to claim 1, wherein the hydraulic oil composition has a ratio (A/B) of (A) a kinematic viscosity (unit: mm²/s) at 80° C. to (B) a high shear viscosity (unit: mPa·s, shear condition: 10⁶/s) at 80° C. of 1.15 or lower. 